Add screenshield and unlock dialog to ctrl-alt-tab

As we now allow the ctrl-alt-tab popup on the lock screen, it should
be possible to navigate back from the top bar, so add the corresponding
elements to the switcher.

https://bugzilla.gnome.org/show_bug.cgi?id=688202

js/ui/screenShield.js

@@ -398,6 +398,7 @@ const ScreenShield = new Lang.Class({
                                       Lang.bind(this, this._onLockScreenKeyRelease));
         this._lockScreenGroup.connect('scroll-event',
                                       Lang.bind(this, this._onLockScreenScroll));
+        Main.ctrlAltTabManager.addGroup(this._lockScreenGroup, _("Lock"), 'changes-prevent-symbolic');
 
         this._lockScreenContents = new St.Widget({ layout_manager: new Clutter.BinLayout(),
                                                    name: 'lockScreenContents' });